An electrophotographic image forming apparatus having an intermediate transfer belt forms an electrostatic latent image on a photoreceptor, then develops a toner image from the electrostatic latent image by use of toners, then transfers the toner image onto the intermediate transfer belt, then further transfers the toner image onto a sheet of recording paper, and finally fixes the toner image onto the sheet of recording paper.
Such transfer of a toner image from the intermediate transfer belt onto a sheet of recording paper is carried out by a second transfer section in which second transfer rollers faces each other across the intermediate transfer belt. Specifically, the second transfer rollers are provided so as to detach from and come into contact with the intermediate transfer belt. The transfer of a toner image is carried out in such a manner that while the second transfer rollers are pressed against the intermediate transfer belt, a sheet of recording paper is carried therebetween so that a bias voltage which causes the toner image to be transferred from the intermediate transfer belt to the sheet of recording paper is applied to the second transfer rollers.
As shown in Patent Literatures 1 and 2, a charging characteristic etc. of the photoreceptor of such an image forming apparatus are changed due to changes of environmental conditions such as temperature and humidity. As a result, an image quality are changed. In view of this, an image forming apparatus carries out process control for image quality adjustment, e.g., every time printing is carried out with respect to a predetermined number of sheets of recording paper, or in accordance with changes of environmental conditions such as temperature and humidity.
In the process control, first, many small toner images (hereinafter, referred to as toner patches) are formed in a line so as to have gradually-changed color densities, with respect to each of toners having respective colors. Then, the color densities of the toner patches on the intermediate transfer belt are measured by a density sensor. On the basis of the measurement result, a control process for image quality adjustment is carried out. In the process control, e.g., an applied voltage to be applied to a charger which charges the photoreceptor and an output value of an exposure light source which exposes the photoreceptor are controlled so that an image quality is adjusted.